The present invention relates to a compound semiconductor light emitting device and, more particularly, to a compound semiconductor light emitting device using a II-VI compound semiconductor and having a novel structure which is suitable for a light emitting device.
A compound semiconductor light emitting device in the wavelength region of an infrared or red visible region having a hetero-junction structure consisting of a III-V compound semiconductor such as In.sub.1-x Ga.sub.x P.sub.1-y As.sub.y /GaAs and Ga.sub.1-x Al.sub.x As/GaAs (0&lt;x, y&lt;1) is conventionally used. A III-V compound semiconductor, however, is unsuitable for a semiconductor laser for a shorter wavelength region such as a green or blue wavelength region mainly because the intrinsic energy gap of the material is too small.
As a countermeasure, development of a light emitting device using a II-VI compound semiconductor has been attempted. For example, J.Appl. Phys. 57 (6), (1985) reports on page 2210 to 2216 that light emission in a blue wavelength region has been obtained from a p-n junction formed by diffusing Ga in bulk ZnSe. The formation of a p-n junction on ZnSe is also described in Japanese Patent Publication Nos. 5337/1987 and 5338/1987.
A II-VI compound, which generally has a large energy gap, is expected as a material for realizing a device for emitting light in a green to blue light emitting wavelength region. In most of II-VI compounds, however, the control of a p-n junction by the diffusion of impurities is almost impossible due to the self-compensation. Therefore, either a p-type or n-type semiconductor is only formed from a II-VI compound except CdTe, and it is very difficult to form both p-type and n-type regions in one semiconductor crystal.
A semiconductor having a strong ionicity such as most of II-VI compound semiconductors is unipolar. In other words, such a crystal is either n or p conduction type. If impurities easy to ionize enter a crystal having a strong ionicity and only one conduction type, a lattice defect (vacancy) is ionized to the opposite charge so as to retain the electric neutrality in the interior of the crystal. The electrons and holes generated by the introduction of the impurities and the ionization of the lattice defect caused thereby combine with each other and emit energy (Er). In a crystal having a strong ionicity, the formation enthalpy (.DELTA.Hv) is smaller than Er in comparison with a crystal having a weak ionicity, so that as impurities are introduced more, the lattice defects increase, thereby substantially completely compensating the carriers generated by the addition of the impurities.
Whether the semiconductor is a p or n conduction type is determined by the ionizing energy of a vacancy and if a vacancy of an element of the II group is an acceptor and a vacancy of an element of the VI group is a donor, the ionizing energy of a vacancy has some relationship with the dimension of the vacancy (see Phys. Rev. 136, 3A (1964) page A826 to A832). A small ionized vacancy has a smaller ionizing energy than a large ionized vacancy, so that the self-compensation increases. For example, in the case of ZnS, the compensation of the S vacancy for p-type impurities is larger than the compensation of Zn vacancy for the n-type impurities, but in the case of ZnTe, since the Te vacancy has a large size, the compensation of the Zn vacancy for n-type impurities is larger than the compensation of Te vacancy for the p-type impurities. Actually, ZnS is an n-type unipolar compound, while ZnTe is a p-type unipolar compound. Such a phenomenon is called charge-compensation or self-compensation.
It has recently been reported a p-n junction is formed by diffusing Ga in bulk p-type ZnSe and bluelight emission is obtained, as described above. In this case, however a broad light emission peak is also realized in a red region other than the blue light region, thereby disadvantageously lowering the efficiency of blue light emission. In order to produce a light emitting device having a good efficiency, it is essential to form a thin film p-n junction having a lattice matching, preferably a p-n homojunction, but no report has been made on such technique.
Additionally, the relationship between the introduction of impurities and a lattice defect density in a III-V compound semiconductor is described in "Oyobutsuri" (Applied Physics), Vol 57, No. 2 (1988), page 216 to 220.